In geophysical prospecting, seismic operations are frequently performed to generate and collect information indicative of the physical properties of subsurface formations. A conventional seismic land operation includes placing a seismic cable along the surface of the earth. Usually, the cable extends in a straight line and contains a plurality of wire pairs. A typical cable will contain from 48 to about 120 wire pairs. At regular intervals along the length of the cable, there are provided electrical connectors generally referred to as "takeouts." Each takeout is electrically connected to a separate pair of wires in the seismic cable. Typically, the takeouts are located at intervals of 50 to 200 feet along the length of the cable.
An array of geophones is attached to each takeout of the seismic cable. The array of geophones may consist of from one to fifty or more individual geophones. Each geophone is typically affixed to the surface of the earth by means of a spike so that, as the earth moves, the geophone moves as well. The geophones in any individual group are electrically interconnected so that the output signal from the group is a measure of the average motion sensed by all the geophones in that specific group.
In a typical seismic survey for a large geographical area, the area is covered by a plurality of survey lines. Seismic profiles are then recorded along these survey lines. On land, a seismic cable, typically approximately two miles in length and formed from a series of identical sections, is laid on the ground along a survey line. At each takeout, there is connected a two-wire conduit to electrically interconnect, for example, about 30 geophones into a single group or array. Each geophone group is located in the general vicinity of the takeout of the seismic cable to which it is attached.
A seismic signal is generated in the vicinity of the geophone groups. Typical seismic sources include explosive charges, weight drops, or other impact sources which impart a sudden impulse of energy to the surface of the earth in the survey area. Another type of seismic source is a vibrator which imparts a vibratory sweep of energy to the earth's surface for a period of time from a few seconds to about 20 seconds. Typically, the seismic signal is imparted into the earth at regularly spaced intervals along a portion of the length of the seismic cable.
One end of the seismic cable is connected to a seismic recorder, which typically includes a magnetic tape recorder. The seismic waves from each group of geophones usually are recorded on the magnetic tape in digital form. A multiple position switch, referred to as a roll-along switch, in a recording truck is advanced to a new position, thereby advancing the portion of the cable connected to the data recording equipment. After the desired data have been recorded, one or more of the cable sections are disconnected from one end of the two mile length of cable and reconnected to the other end. A new data recording cycle is then undertaken. After recordings have been taken along the full length of one survey line, the cable, geophones and recorder are moved to the next survey line and the process repeated until the entire geographical area has been covered.
The foregoing type of seismic data acquisition is generally referred to as the reflection method in which the seismic waves or impulses are generated at or near the earth's surface and these waves are reflected from subsurface acoustic impedance boundaries and detected by the array of seismic detectors located at the surface. Detector groups of considerable length are used preferentially in reflection seismic exploration in order to discriminate between signals and unwanted noise.
A disadvantage of having long lengths of detectors is the attenuation of high frequency signals. Attempts have been made to improve the high frequency response of seismic rays by using very short (a smaller number of detectors) arrays. The most common result is a considerable degradation of data quality owing to the decrease in signal-to-noise ratio.
It has been considered that it might be possible to restore the signal-to-noise ratio to a value comparable to that of arrays in common use by greatly increasing the number of arrays, in effect retaining the number of individual detectors in common use but subdividing them into many more but shorter arrays. A difficulty resulting from the use of shorter groups of sensors is that, for example, if a full-length, 2-mile seismic cable is to be employed and if the sensor density (number of sensors) is to remain unchanged, the number of signal channels which must be connected to the recorder is increased by an order of magnitude as the group length is reduced. This would mean about 10 times as many conductor wire pairs would have to be added to the cable if the group lengths were to be significantly reduced. Such an increase in the number of conductor wire pairs would greatly increase the weight and the bulk of the cable sections and decrease their flexibility to unacceptable levels.